1. Preheat the oven to 400°F.
2. Slice the leeks. Place leeks, oil, butter, salt and pepper in the bowl fitted with the mixer. Cook at speed 3 at 270°F for 20 minutes.
3. Add cream and 2 beaten egg yolks. Launch at speed 3 at 185°F for 5 minutes. 4. Roll out two 12-inch circles of shortcrust pastry. Place one crust in a buttered tart pan. Spread the leek mixture in the crust and cover with the second pastry. Pinch the edges to seal well and brush the top with remaining beaten egg yolk.
5. Bake for 32-35 minutes or until light brown.